001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *     http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.configuration2.tree;
018
019import java.util.List;
020
021/**
022 * <p>
023 * An extension of the {@link NodeHandler} interface which allows access to so-called <em>references</em> stored for a
024 * node.
025 * </p>
026 * <p>
027 * Some specialized configuration implementations needs to store additional data for the nodes representing
028 * configuration properties. This interface provides methods for querying this data. For instance, it is possible to
029 * query a reference object stored for a specific node.
030 * </p>
031 * <p>
032 * {@link InMemoryNodeModel} supports references. It can be queried for a {@code ReferenceNodeHandler} which can then be
033 * used for dealing with references.
034 * </p>
035 *
036 * @since 2.0
037 */
038public interface ReferenceNodeHandler extends NodeHandler<ImmutableNode> {
039    /**
040     * Gets the reference object associated with the specified node. If no reference data is associated with this node,
041     * result is <b>null</b>.
042     *
043     * @param node the node in question
044     * @return the reference object for this node or <b>null</b>
045     */
046    Object getReference(ImmutableNode node);
047
048    /**
049     * Returns a list with the reference objects for nodes which have been removed. Whenever a node associated with a
050     * reference object is removed from the nodes structure managed by the owning model, the reference object is recorded.
051     * This is necessary for instance to free some resources. With this method all recorded reference objects can be
052     * queried. They are typically returned in the order in which they have been removed.
053     *
054     * @return a list with reference objects for nodes removed from the model
055     */
056    List<Object> removedReferences();
057}